1. Field of the Invention
The present invention relates to a switching circuit for varying rotational speed of a motor, especially of a power driven tool such as a power driven screwdriver and a power driven drill.
2. Description of the Related Art
A switching circuit for controlling rotational speed of a motor is shown in FIG. 5. The switching circuit includes a power supply line 30 connecting a DC power source E with a DC motor M. In the power supply line 30, there is provided a change-over switch 32 for changing the direction of rotation of the DC motor M from the forward direction to the reverse direction or vice verse, a parallel connection of an on-off switch 34 with a short circuiting switch 36, a triangle wave generator 42 connected with the terminals of the DC motor M via diodes 38, 40 which are forwardly connected, a comparator 46 for comparing a control signal a derived from the triangle wave generator 42 with a rotational speed setting signal b derived from a variable resistor 44, and a final control circuit 48 for controlling the on-off switch 34 based on an output signal c of the comparator 46.
The variable resistor 44 changes its output or the voltage of the setting signal b according to the amount of operation of a control member (not shown) which may cooperate with a trigger of a power driven tool. The voltage of the setting signal b becomes lower as the amount of operation of the control member increases. The compartor 46 provides the output signal c when the voltage of the control signal a is higher than that of the setting signal b, and the final control circuit 48 turns on the on-off switch 34 based on the output signal c As the amount of operation of the control member increases, the time during which the on-off switch 34 is on becomes longer or the duty factor increases, so that the rotational speed of the DC motor M continuously increases.
Further, when the amount of operation of the control member becomes maximum, the short circuiting switch 36 turns on and the voltage of the power source E is directly applied to the DC motor M, so that the DC motor continuously rotates at the maximum speed.
The on-off switch 34 usually includes a power transistor which is to be used at the duty factor of less than 50%. When the short circuiting switch 36 is turned on, the output signal c corresponding to the duty factor of about 50% is continuously supplied to the final control circuit 48, and the on-off switch 34 turns on at the duty factor of about 50%. As explained above, the on-off switch 34 and the short circuiting switch 36 are connected in parallel in the power supply line 30. Therefore, when the on-off switch 34 is on, the current flows through both the on-off switch 34 and the short circuiting switch 36. On the other hand, when the on-off switch 34 is off, the current flows through only the short circuiting switch 36. For example, if the voltage of the DC power source E is 10 V, this voltage is not directly applied to the short circuiting switch 36 when the on-off switch 34 is on. However, this voltage of 10 V is directly applied to the short circuiting switch 36 when the on-off switch 34 is off, so that a spark is produced at the contact point of the short circuiting switch 36 because of the substantial potential difference by such direct application. Thus, when the short circuiting switch 36 turns on, the on-off switch 34 is intermittently turns on at the duty factor of about 50%. Therefore, as shown in FIG. 6, the short circuiting switch 36 may turn on either at a timing T1 where the control signal c exists and the on-off switch 36 is on or at a timing T2 where the control signal c does not exist and the on-off switch 34 is off. When the short circuiting switch 36 turns on at the timing T2, the voltage of the DC Power source E is directly applied to the short circuiting switch 36, so that the spark is produced at the short circuiting switch 36. Such production of sparks also occurs when the short circuiting switch 36 turns off from on, and as sparks are repeatedly produced, the contact is worn and the life of the short circuiting switch 36 is shortened.
In order to overcome such drawbacks, it may be considered to use as the on-off switch 34 a power transistor which permits large duty factor such as 100%, so that the on-off switch 36 can always be on when the short circuiting switch 36 is turned on. However, such a power transistor to permit large duty factor is expensive and therefore, the costs of manufacturing the switching circuit increases, thereby producing another drawback.